Each of the three fascicles has its own motorneuron subnucleus in the motor column in the spinal cord. The medial head is formed predominantly by small type I fibers and motor units, the lateral head of large type IIb fibers and motor units and the long head of a mixture of fiber types and motor units.[2][3] It has been suggested that each fascicle "may be considered an independent muscle with specific functional roles."[2]

The fibers converge to a single tendon to insert onto the olecranon process of the ulna (though some research indicates that there may be more than one tendon)[4] and to the posterior wall of the capsule of the elbow joint where bursae (cushion sacks) are often found. Parts of the common tendon radiates into the fascia of the forearm and can almost cover the anconeus muscle.[1]

All three heads of the triceps brachii are classically believed to be innervated by the radial nerve.[5] However, a study conducted in 2004 determined that, in 20 cadaveric specimens and 15 surgical dissections on participants, the long head was innervated by a branch of the axillary nerve in all cases.[6]

A tendinous arch is frequently the origin of the long head and the tendon of latissimus dorsi. In rare cases, the long head can originate from the lateral margin of the scapula and from the capsule of the shoulder joint.[1]

The triceps is an extensor muscle of the elbow joint and an antagonist of the biceps and brachialis muscles. It can also fixate the elbow joint when the forearm and hand are used for fine movements, e.g., when writing. It has been suggested that the long head fascicle is employed when sustained force generation is demanded, or when there is a need for a synergistic control of the shoulder and elbow or both. The lateral head is used for movements requiring occasional high-intensity force, while the medial fascicle enables more precise, low-force movements.[2]

With its origin on the scapula, the long head also acts on the shoulder joint and is also involved in retroversion and adduction of the arm.[1]

It is sometimes called a three-headed muscle (Latin literally three-headed, tri - three, and ceps, from kaput - head), because there are three bundles of muscles, each of different origins, joining together at the elbow. Though a similarly named muscle, the triceps surae, is found on the lower leg, the triceps brachii is commonly called the triceps.

Historically, the plural form of triceps was tricipites, a form not in general use today; instead, triceps is both singular and plural (i.e., when referring to both arms).

In the horse, 84%, 15%, and 3% of the total triceps muscle weight correspond to the long, lateral and medial heads, respectively.[7]

Many mammals, such as dogs, cattle, and pigs, have a fourth head, the accessory head. It lies between the lateral and medial heads.[2] In humans, the anconeus is sometimes loosely called "the fourth head of the triceps brachii".